Artificial skin and elastic strain sensor

ABSTRACT

An elastic strain sensor can be incorporated into an artificial skin that can sense flexing by the underlying support structure of the skin to detect and track motion of the support structure. The unidirectional elastic strain sensor can be formed by filling two or more channels in an elastic substrate material with a conductive liquid. At the ends of the channels, a loop port connects the channels to form a serpentine channel. The channels extend along the direction of strain and the loop portions have sufficiently large cross-sectional area in the direction transverse to the direction of strain that the sensor is unidirectional. The resistance is measured at the ends of the serpentine channel and can be used to determine the strain on the sensor. Additional channels can be added to increase the sensitivity of the sensor. The sensors can be stacked on top of each other to increase the sensitivity of the sensor. In other embodiments, two sensors oriented in different directions can be stacked on top of each other and bonded together to form a bidirectional sensor. A third sensor formed by in the shape of a spiral or concentric rings can be stacked on top and used to sense contact or pressure, forming a three dimensional sensor. The three dimensional sensor can be incorporated into an artificial skin to provide advanced sensing.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims any and all benefits under law including benefitunder 35 U.S.C. §119(e) of the U.S. Provisional Application No.61/538,841, filed Sep. 24, 2011, the contents of which are incorporatedherein by reference in its entirety.

This application is related to U.S. Application Ser. No. 61/387,740,filed on Sep. 29, 2010 bearing Attorney Docket No. 002806-010099, whichis hereby incorporated by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with US government support under grant no. CNS0932015 awarded by the National Science Foundation. The US governmenthas certain rights in the invention.

REFERENCE TO MICROFICHE APPENDIX

Not Applicable

BACKGROUND

1. Technical Field of the Invention

The present invention is directed to elastic strain and pressure sensorsand associated devices and systems for measuring motion and contact.Specifically, the present invention is directed to a hyper-elasticstrain sensor that can be used to create an artificial skin that measuremotion and touch.

2. Description of the Prior Art

Emerging technologies such as wearable computing [1] and soft activeorthotics [2] will depend on stretchable sensors that registerdeformation and surface pressure. These softer-than-skin sensors mustremain functional when stretched to several times their rest length,avoid hysteresis and permanent deformation, and preserve the naturalmechanics of the wearer or host system. Hyper-elastic transducers forstrain and pressure sensing represent just one aspect of the muchbroader and potentially revolutionary fields of elastically stretchableelectronics and computing.

Current approaches to stretchable electronics include buckled (wavy)films of semiconductors for stretchable circuits and diodes [3-5] aswell as elastomers that are embedded with microchannels of conductiveliquid [6-8]. The latter approach utilizes many of the same molding,embossing and lithography techniques that are used to fabricate softmicrofluidic devices [9-11]. One advantage of elastomers is theirhyper-elasticity, which allows for mechanical durability and stretchesas great as 1000%. Such properties are particularly favorable inwearable devices such as adaptive orthotics and insoles that mustsustain large deformations and pressures.

Previous efforts in soft pressure and strain sensing and so calledartificial skin include capacitive sensors composed of an elasticinsulator layered between conductive fabric [12-14] or a silicone rubbersheet embedded with thin gold film [15]. Other efforts include resistivesensors composed of elastomer embedded with conductive microparticlefiller [16-18] or ionic liquid [19-21] and a flexible artificial skinembedded with semiconductor nanowires [22].

Prior designs for pressure sensing are adapted from the Whitney straingauge, which was introduced in 1949 to measure the change incircumferential girth of muscles and limbs [23, 24]. The originalWhitney strain gauge was composed of a rubber tube filled with mercuryand used a Wheatstone bridge to measure the change in electricresistance of the mercury channel corresponding to stretch. Recently,this principle has been extended to stretchable microelectronics,composed of eGaIn-filled microchannels embedded in polydimethylsiloxane(PDMS) rubber [6]. Embedded channels of eGaIn can also operate as astretchable, mechanically tunable antenna [7] or as strain sensors [8]for measuring stretches of as much as 200%.

SUMMARY

The present invention is directed to a stretchable or elastic strainand/or pressure transducer composed of a flexible material embedded withconductive liquid in an array of microchannels. Pressing the surface orpulling the flexible elastomer material deforms the cross-section of thechannels and changes the electric resistance of the conductive liquid inthe microchannels.

The present invention is also directed to elastic sensors that respondto strain in a single direction. This can be accomplished by forming aset of elongated microchannels, each extending substantially parallel toa strain axis. The microchannels can be interconnected at their ends byloop portions to form a continuous channel over which to measureelectrical resistance. In accordance with some embodiments of theinvention, the loop portions can that have sufficient cross-sectionalarea in a direction transverse to the strain axis that strain in adirection transverse to the strain axis does not result in significantchange in electrical resistance of the sensor, thus enablingunidirectional sensing. In these embodiments, the elastic sensors can bepositioned to measure strain in one direction and multiple elasticsensors according to the invention can combined in differentorientations to measure strain in two or more dimensions.

The present invention is also directed to elastic sensors that includeembedded eGaIn channels that also operate as pressure sensors with 1 kParesolution and 0-100 kPa range. In contrast to strain sensing, themechanics of pressure sensing are complex and involve the use ofelasticity and contact mechanics to derive a predictive mathematicalmodel for describing the relationship between external pressure andelectrical conductivity. In addition, the embedded microchannels can beproduced using a maskless fabrication method that combines direct laserwriting [25, 26] with soft lithography [9, 27] to produce micron-orderfeature sizes.

The present invention is directed to elastic sensors that can be formedin a compact package. The microchannels according to the invention canbe closely spaced together in a horizontal plane as well as stackedvertically. This provides for highly sensitive sensor with a small,flexible, form factor. These sensor configurations can be fabricated ina skin that can be applied to robotic or orthopedic applications wherejoint position and motion sensing is needed.

These and other capabilities of the invention, along with the inventionitself, will be more fully understood after a review of the followingfigures, detailed description, and claims.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows an elastic strain sensor according to an embodiment of theinvention.

FIG. 2 shows a close-up view of the loop portions of an elastic strainsensor according to the embodiment of the invention shown in FIG. 1.

FIG. 3 shows the calibration plot of an elastic strain sensor accordingto an embodiment of the invention.

FIG. 4 shows an elastic pressure sensor according to an embodiment ofthe invention.

FIG. 5 shows a method of fabricating an elastic strain sensor accordingto an embodiment of the invention.

FIG. 6 shows a diagrammatic view of a multi-modal elastic strain sensoraccording to an embodiment of the invention.

FIG. 7 shows a multi-modal elastic strain sensor according to anembodiment of the invention.

FIG. 8 shows a method of fabricating a multi-modal elastic strain sensoraccording to an embodiment of the invention.

FIG. 9 shows layered views of a multi-modal elastic strain sensoraccording to an embodiment of the invention.

FIGS. 10A and 10B show a circuit diagram and test configuration fortesting a multi-modal elastic strain sensor according to an embodimentof the invention.

FIG. 11 shows graphs of the strain and sensor output of a multi-modalelastic strain sensor according to an embodiment of the invention.

FIG. 12 shows an angle measurement of a single d.o.f. robotic arm usinga hyper-elastic strain sensor according to an embodiment of theinvention.

FIG. 13 shows an angle measurement of a multi-d.o.f. robotic arm using ahyper-elastic strain sensor according to an embodiment of the invention.

FIG. 14 shows a stretchable body suit for measuring body joint anglesusing one or more hyper-elastic strain sensor according to an embodimentof the invention.

FIG. 15 shows a stretchable glove with strain sensors for measuringfinger joint angles using one or more hyper-elastic strain sensoraccording to an embodiment of the invention.

FIG. 16 shows a plot of the change in electrical resistance as afunction of applied pressure according to an embodiment of theinvention.

FIG. 17 shows a plot of the change in electrical resistance as afunction of lateral displacement (x) according to an embodiment of theinvention.

FIG. 18 shows a two-dimensional, plane strain representation ofelastomer embedded with a microchannel of width w and height h accordingto an embodiment of the invention. The surface of the elastomer issubject to a pressure p uniformly distributed over a width a.

FIG. 19 shows a plot of the change in electrical as a function of depthz of the sensor according to an embodiment of the present invention.

FIGS. 20A and 20B show instrumentation and placement of sensors in priorart systems for determining joint angle.

FIG. 20C shows a flexible system for measuring joint angle according toone embodiment of the invention.

FIG. 21 shows a system for measuring forces and motion of the footaccording to one embodiment of the invention.

FIGS. 22A-22E show a sensory system according to one of the embodimentsof the invention used in various applications.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention is directed to elastic sensors and methods forfabricating elastic sensors that respond to strain in a singledirection. This can be accomplished by forming a set of elongatedmicrochannels in an elastic material such as silicone rubber sheet(EcoFlex 0030, SmoothOn, Easton, Pa.; PDMS, Dow Corning). Eachmicrochannel can be formed to extend substantially parallel to a strainaxis and the microchannels can be interconnected at their ends by loopportions to form a continuous channel over which to measure electricalresistance. The continuous channel can be filled with a conductivematerial, such as a conductive liquid, for example, non-toxic eutecticgallium-indium (eGaIn, BASF). In accordance with some embodiments of theinvention, the loop portions can that have sufficient cross-sectionalarea in a direction transverse to the strain axis that strain in adirection transverse to the strain axis does not result in significantchange in electrical resistance of the sensor and enables unidirectionalsensing. In these embodiments, the elastic sensors can be positioned tomeasure strain in one direction and multiple elastic sensors accordingto the invention can combined in different orientations to measurestrain in two or more dimensions.

FIG. 1 and FIG. 2 show an elastic strain sensor 100 according to anembodiment of the present invention. The strain sensor 100 can be formedfrom a flexible elastic substrate material 102 by molding or etching toform elongated microchannels 110 and loop portions 120. Themicrochannels 110 and the loop portions 120 can be filled with aconductive liquid 130 and the strain on the sensor 120 can be determinedfrom changes in the electrical resistance of the conductive liquid 130as the elastic material and the conducting liquid are stretched. Theloop portions 120 connect adjacent ends of the elongated microchannels110 to form a serpentine channel that extends from a first connectionreservoir 132 to a second connection reservoir 134. The first connectionreservoir 132 and the second connection reservoir 134 can be used toinject the conductive liquid 130 so that it flows through each of themicro channels 110 and can receive wires to connect to the controlsystem that measure the electrical resistance of the full extent of thecombined microchannels 110 and loop portions 120.

FIG. 2 shows an enlarged view of the loop portions 120 of the strainsensor 100. The loop portions 120 can be substantially larger than themicrochannels 110 in the transverse direction, so that strain applied ina direction transverse to the strain axis 104 does not result inappreciable increase in electrical resistance. In this way, the elasticstrain sensor 100 can be sensitive to strain applied along the strainaxis 104 or strain having a component that extends along the strain axis104 and insensitive to strain transverse to the strain axis 104.

Each of the microchannels 110 can be formed with a uniform cross-sectionand filled with the conducting liquid, such as eutectic gallium-indium(eGaIn) available from BASF, Florham Park, N.J. Each of themicrochannels 110 can be substantially straight, zig-zag, or S shaped.In accordance with one embodiment, the microchannels 110 can beconnected end to end by the loop portions 120 such that a singlecontinuous channel filled with the conducting liquid 130 can be formed.In this embodiment, each microchannel extends parallel to the strainaxis and when the elastic material is subject to strain, each of themicrochannels, along with the conductive liquid 130 carried therein, canbecome elongated increasing the electrical resistance. One advantage ofthe present invention is that each microchannel becomes elongatedcausing the overall length of the channel of conducting liquid to beelongated in per portion to the number of microchannels. Addingmicrochannels can be used to increase the sensitivity of the strainsensor. In some embodiments, the sensor can include 36 or moremicrochannels 110, and each microchannel 110 can be 250 μm wide by 250μm high and the loop portions can be 1.0 mm wide by 250 μm high.

Each of the microchannels 110 can be connected by loop portions 120 thatforms a continuous channel that serpentines over a surface. Inaccordance with some embodiments of the invention, the loop portions 120can be substantially larger in cross-sectional area than the unstrainedmicrochannels 110 such that strain in a direction transverse to thestrain axis 104 does not cause significant change in electricalresistance. In this embodiment, the elastic stain sensor 100 becomesunidirectional along the strain axis 104.

The elastic strain sensor 100 can be formed from any elastic materialincluding silicone and rubber materials (e.g., EcoFlex0030 andEcoFlex0050, SmoothOn, Easton, Pa.; PDMS, Dow Corning, Midland, Mich.;P-20 and GI-1120, Innovative Polymers, Saint Johns, Mich.; Tap PlatinumSilicone System, Tap Plastics, Calif.) and soft polyurethane materials(e.g., Dragon Skin, SmoothOn, Easton, Pa.; IE-35A, Innovative Polymers,Saint Johns, Mich.). In accordance with one embodiment, a low viscosity(3000 cps) mixture of EcoFlex can be used to reproduce the fine featuresof the mold.

In general, the process for fabricating the sensor can include moldmaking, casting the layer(s), bonding the layers together and injectingthe conducting liquid. In accordance with one embodiment of theinvention, the elastic strain sensor 100 can be produced by casting thesilicone material into one or more 3D printed molds (e.g., Connex 500,Objet Geometries Ltd.). In this embodiment, one layer can be patternedto form the microchannels 110, loop portions 120 and connectionreservoirs 132, 134 and the other layer is not patterned, providing anessentially flat layer to be bonded to the patterned layer. After curingunder ambient conditions for approximately 4 h, the elastomer layers canbe removed from the molds and bonded together with a thin, uncured layerof the elastomer material (e.g., EcoFlex).

To avoid filling the exposed microchannels, the unpatterned elastomermold can be first spin-coated with a thin, uncured layer (e.g., 1100 rpmfor 45 s) of elastomer, which can be then partially cured, for example,for 30 s at 60° C. in a convection oven. The patterned elastomer moldcan then be gently bonded to the unpatterned surface. The two elastomerlayers (the unpatterned smooth sheet and the patterned sheet containingthe exposed microchannels) can be cured together under ambientconditions for several hours. After the sheets are sufficiently bondedtogether, a syringe can be used to fill each channel with eGaIn. Lastly,the ends of the channel can be sealed with a final coating of uncuredelastomer material.

In accordance with another embodiment of the invention, sensors withmicrochannels in the 20-300 micrometer range can be fabricated bycasting an elastomer material (e.g., PDMS) in an SU-8 mold that ispatterned by maskless soft lithography. Photoresist (SU-8 2050) can bespun onto a clean silicon wafer, for example, at 500 rpm for 10 s(spread), followed by 4000 rpm for 30 s (spin). The wafer can then beplaced on a hot plate at 65° C. for 3 min and 95° C. for 6 min. Next,the coated wafer can be patterned via direct-write laser exposure [25,26] using a diode pumped solid-state (DPSS) 355 nm laser micromachiningsystem. The system can be calibrated to expose a 20 μm thick SU-8coating to produce channels with a range of 25 to 1000 μm in width and≧50 μm spacing. The wafer can be post-baked on a hot plate, for example,at 65° C. for 1 min and 95° C. for 6 min, and consequently developed for5 min in SU-8 developer. In order to allow for easier removal insubsequent molding steps, a hydrophobic monolayer can be introducedthrough vapor deposition. The patterned wafer can be placed in anevacuated chamber (20 mTorr) with an open vessel containing a few dropsof trichloro (1H,1H,2H,2H-perfluorooctyl) silane (Aldrich) for 3 h.Next, the PDMS (Sylgard 184; Dow Corning, Midland, Mich.) can be cast inliquid form (e.g., 10:1 mass ratio of elastomer base to curing agent)against the silicon wafer. PDMS can then be partially cross-linked inthe mold by oven-curing at 60° C. for 30-40 min. The microchannels canbe constructed by bonding patterned PDMS to unpatterned PDMS via oxygenplasma treatment (Technics Plasma Stripper/Cleaner; 60 W for 30 s). Thesealed microchannels can be completely cured, for example, at 60° C.overnight. Finally, the microchannels can be filled with eGaIn usingconventional tubing and syringe dispensing.

Additional layers with patterned elastomer (e.g., PDMS) can be formed bymolding the patterned layer with an unpatterned back surface. Theunpatterned back surface can similarly be bonded to an additionalpatterned layer. In some embodiments, the microchannels for each layercan be aligned with a common axis and connected by holes or openings inthe patterned layer(s) that provides an interconnect between layers. Insome embodiments, the microchannels for each layer can be isolated so asto provide more than one sensor. In some embodiments, the microchannelsfor each layer can extend along intersecting axes to allow strainsensing in multiple directions.

In accordance with one embodiment of the invention, a strain sensor wasformed with 36 channels, each channel being 250 μm wide by 250 μm highand the loop portions being 1.0 mm wide by 250 μm high. This device hada nominal resistance at rest of 5.8 ohms. The gauge factor of the straingauge can be determined:

$\begin{matrix}{\frac{\Delta \; R}{R_{0}} = {{G\; ɛ} + {\alpha \; \theta}}} & (1)\end{matrix}$

Where ΔR is the resistance change, R is the resistance at rest, ε is thestrain, α is the temperature coefficient and θ is the temperaturechange. Assuming there is no temperature effect, the gauge factor wasdetermined empirically to be 3.04. FIG. 3 shows that the plot of thechange in resistance and the strain is substantially linear withoutconsidering temperature effects.

FIG. 4 shows a pressure sensor 400 according to an embodiment of thepresent invention. In this embodiment, the pressure sensor 400 can beformed from a flexible elastic substrate material 402 by molding oretching to include a circular microchannel 410 formed in a spiral or setof concentric loops. The circular microchannel 410 can be filled with aconductive liquid 430 and the pressure on the sensor 400 can bedetermined from changes in the resistance of the conductive liquid 430as the elastic material and the conductive liquid is compressed.

In accordance with one embodiment of the invention, a straightmicrochannel filled with eGaIn material can be used for simultaneouslymeasuring applied pressure and electrical resistance. The ends of theeGaIn-filled channels can be wired to a precision multimeter (Fluke8845A). A rigid glass rectangle of width a and length L can be pressedinto the sensor with a digital height gauge (Swiss PrecisionInstruments, Inc.). In order to distribute the pressure more uniformlyand better simulate tactile or elastic contact, a 5 mm thick sheet ofelastomer with the same area as that of the glass rectangle can beinserted between the glass and the sensor surfaces. The sensor can besupported by an electronic scale (6000 g OHAUS Scout Pro) that measuresthe total force F exerted on the surface. The average pressure exertedover the area of contact can be calculated as p=F/aL.

The change in electrical resistance ΔR of the embedded, conductiveliquid-filled channels can be determined empirically as a function ofthe applied pressure p using an experimental setup. Both experimentallymeasured values (open circles) and theoretical predictions (solid curve)are plotted in FIG. 16 for an elastomer containing a straight channelwith width w=2 mm, height h=1 mm, and with a top face that is at adistance z=2 mm from the surface of the elastomer. Pressure is appliedover an area of length L=27 mm and width a=25 mm. For this set ofmeasurements, the major axis of the contact area (which has length L)can be aligned with the centerline of the channel. The plot containsdata points from multiple loading and unloading cycles, demonstratingsignificant repeatability and low hysteresis.

As shown in the plot in FIG. 16, the change in electrical resistance ΔRincreases exponentially with applied pressure. This curve closelymatches the theoretical prediction, which is represented by the solidline. It is important to note that no data fitting is used; thetheoretical curve is derived entirely from the prescribed geometry, theprescribed pressure, the known resistivity p=29.4×10-8 Ωm-1 of eGaIn [6]and the elastic modulus E=125 kPa of the rubber, which are independentlymeasured by comparing the pressure with the depth of indentation. Theclosed-form theoretical solution and derivation are presented in thenext section.

As expected, ΔR decreases the further the center of applied pressuremoves from the channel. FIG. 17 presents a plot of ΔR versus lateraldisplacement x for pressures p=15 kPa and p=25 kPa. As illustrated inFIG. 18, x is defined as the horizontal distance between the channelcenterline and the major axis of the contact area. For both pressures,the signal ΔR decreases significantly with increasing relativedisplacement. The theoretical predictions, which are represented bydashed and solid curves for 15 and 25 kPa, respectively, are reasonablyconsistent with experimental measurements, which correspond to the opensquares and circles. While the surface pressure is approximatelyuniform, there are small stress concentrations near the edges of thecontact zone. Hence, when x=4 mm and the channel is between the centerand the edge of the contact zone, the nominal stress is slightly greaterand a larger response ΔR is measured. Lastly, a plot of ΔR versuschannel depth z (for x=0) is presented in FIG. 6. Referring to FIG. 5, zis defined as the distance between the surface of the elastomer and thetop wall of the channel. As demonstrated in the experimental results,the resistance change ΔR decreases the farther the channel is from thesurface. This trend is also predicted by the theory, although the theoryappears to overestimate the absolute change by as much as a factor of 2.

The mechanics of the microchannel embedded elastomer are complex and inaccordance with one embodiment of the invention, can be modeled with anapproximate mathematical analysis. In accordance with this embodiment, atwo-dimensional representation of a straight channel with rectangularcross-section embedded in an elastomeric halfspace can be used toapproximate the microchannel. As illustrated in FIG. 18, the channel haswidth w, height h, and a top wall that is at a distance z from thesurface of the elastomer.

A uniform external pressure p can be applied to the surface of theelastomer over an area of width a. As shown in FIG. 18, the centers ofthe channel and the area of applied pressure are offset horizontally bya distance x. For channels close to the center of the applied pressure(i.e. |x|<a/2 and z<a), elastic deformation will reduce thecross-sectional area and hence increase electric resistance. Thereduction in cross-sectional area is primarily governed by the magnitudeof the vertical component of the stress tensor: σ_(z)=σ_(z)(x, z; p, a).Since the applied pressure is compressive, σ_(z) will have a negativesign. As in the case of crack growth in linear elastic fracturemechanics (LEFM), the field lines of the internal stress σ_(z) willconcentrate about the edges of the microchannel [28, 29]. This is inorder to satisfy the boundary condition of zero traction on the walls ofthe channel. Because the channel is filled with fluid, the walls willnot be traction free but are instead subject to hydrostatic pressure.However, this internal channel pressure is considered negligible incomparison to the induced nominal stress σ_(z) and so zero traction isassumed.

According to LEFM, an average vertical stress σ_(z) applied in thevicinity of a crack will increase the gap between the crack faces by anamount Δh=2(1−v²)wσ_(z)/E, where v is Poisson ratio and E is the elasticmodulus [28]. Because the microchannels are small compared to thedimensions of the elastomer, their influence on the stress distributionwill be negligible except in the immediate vicinity of each channel.Therefore, for channels below the area of contact (|x|<a/2 and z<a), theaverage stress in the neighborhood of the channel may be approximated asσ_(z)=−p. Substituting this into the expression for Δh implies that thetotal change in electrical resistance will be approximately

ΔR=ρL/wh{(1/(1−2(1−v2)wp/Eh))−1}  (2)

In general, p should be replaced with χp, where χ=χ(x, z) is acorrection that depends on the relative position (x, z) of the channelcenterline. The correction χ=−σ_(z)/p can be obtained by solving σ_(z)using Boussinesq's method: [30]

$\begin{matrix}{\sigma_{z} = {\int_{{- a}/2}^{a/2}{\int_{- \infty}^{\infty}{{- \frac{3\; {pz}^{3}}{2\pi}}\left\{ {\left( {x - X} \right)^{2} + Y^{2} + z^{2}} \right\} \ {Y}\ {X}}}}} & (3)\end{matrix}$

A closed-form, elementary expression for σ_(z) can be obtained withMaple 13 (Waterloo Maple Incorporated, 2009). Solving for χ yields

χ=(c ₁ c ₂ −c ₃)/c ₄  (4)

wherec₁=tan⁻¹((a+2x)/2z)+tan⁻¹((a−2x)/2z)c₂=−8x²a²+32x²z²+8z²a²+16x⁴+16z⁴+a⁴c₃=−16zax²+4za³+16z³ac₄=π(4x²+4xa+a²+4z²)(4x²−4xa+a²+4z²).This is used to evaluate the change in electrical resistance as afunction of x and z:

$\begin{matrix}{{\Delta \; R} = {\frac{\rho \; L}{wh}\left\{ {\frac{1}{1 - {2\left( {1 - v^{2}} \right){{wxp}/{Eh}}}} - 1} \right\}}} & (5)\end{matrix}$

In accordance with some embodiments of the invention, the derivedrelations are consistent with experimental measurements for a wide rangeof pressures p and relative positions (x, z). In FIG. 17 and FIG. 19there appears to be close to a 50% discrepancy between the theory andthe experiment. This could be due to the simplifying assumptions of thetheoretical model, which is based on plane strain linear elasticity,ignores the influence of the channel on global stress distribution, andassumes uniform channel collapse with zero surface traction and constantwidth. Relaxing these assumptions can be used determine a more accurateequations that better matches the experimental result. However, thesemodels require numerical computations or finite element analyses thatwill not yield an algebraic closed-form solution, such as the onepresented in equation (5).

In addition to capturing the principal mechanics of the elastomerpressure transducer, the theory reveals several properties that can beexploited for customized functionality. The first property allows formechanical decoupling between pressure sensing and stretch sensing.Thus, a system according to embodiments of the invention can be able todistinguish whether change in microchannel conductivity is induced bypressure or stretching.

The second property relates to the sensor bandwidth, i.e. the range ofpressures that the sensor can detect. Sensor response to pressure andstretch are decoupled by selecting the appropriate microchannel depth zand path (e.g. straight, serpentine and spiral). As demonstrated in FIG.19, the sensor response vanishes as z exceeds the width a of the contactarea. In contrast, the change in electrical resistance due to channelelongation is invariant with microchannel depth. Instead, elongationresponse is governed by the simple formula ΔR/R0=λ²−1, where R0=ρL/wh isthe original resistance of the unstretched channel and the stretchλ=L_(f)/L is the ratio of the stretched length L_(f) to the naturallength L. This implies that a microchannel embedded deep within theelastomer (a distance z>a from the surface for anticipated values of a)will only measure stretch and not pressure.

Alternatively, a spiral-shaped microchannel embedded close to theelastomer surface, as shown in FIG. 1( a), will detect pressure but notuniaxial stretching. This is because increased electrical resistance inone direction is balanced by reduced resistance in the perpendiculardirection.

Sensor bandwidth is controlled by a characteristic pressure {circumflexover (p)}=Eh/w and thus depends only on the elastic modulus E of theelastomer and the aspect ratio h/w of the microchannel cross-section.Noting that R₀=ρL/wh is the natural resistance of the channel, itfollows from equation (2) that for a channel embedded near the surfaceof the elastomer, ΔR/R₀=1/(1−2(1−v²)p/{circumflex over (p)}). Dependingon the ratio p/{circumflex over (p)}, the relative change in electricalresistance can range from fractions of a percent to orders of magnitude.Consider, for example, EcoFlex (E=125 kPa) embedded with a microchannelof width w=100 μm and thickness h=20 μm. In response to a typicalkeystroke pressure in the range of 1-10 kPa, the electrical resistanceof the embedded microchannel would change by an order of 1%. Incontrast, peak pressure in foot-ground contact during walking is in theorder of 100 kPa, which would result in an approximately 50% change inelectrical resistance. For all applications, the design parameters E andh/w should be selected such that the characteristic pressure {circumflexover (p)} is comparable to the range of anticipated pressures p.

FIG. 5 shows a diagram of a method of fabricating a multi-layer strainsensor according to an embodiment of the invention. The method includespreparing the molds for each layer. In this embodiment, two layers ofsensors are provided, so three molds can be used to form three layerselastomer material as shown in FIG. 5( a). After the elastomer materialcures, the cast material is removed from the molds. As shown in FIG. 5(b), layer 0 is the unpatterned layer and can remain in the mold andLayer 1 can include an interconnect to connect the microchannels formedbetween the layers of elastomer material. Layer 0 can be spin coatedwith the elastomer material at 2000 rpm for 50 sec. and then partiallycured at 60 degrees C. for 1 min, as shown in FIG. 5( c). Layer 1 can bebonded to Layer 0 by laminating with a light pressure as shown in FIG.5( d). Layer 2 can be bonded to the top surface of Layer 1 by the sameprocess as Layer 1 was bonded to Layer 0. The top surface of Layer 1 canbe spin coated with the elastomer material and then partially cured, asshown in FIG. 5( e). Additional layers of elastomer material can bebonded using the same process. After the last layer is bonded to theelastic strain sensor, Layer 0 can be removed from the mold, as shown inFIG. 5( f). The conducting liquid 130 can be injected into the channels110 using a syringe. In one embodiment of the invention, more than onesyringe can be used during the injecting process. At least one syringecan be used to inject the conducting liquid, such as eGaIn into oneconnection reservoir while at least one other syringe can be used toremove the rapped air, such as from the other connection reservoir, asshown in FIG. 5( g). After the conducting liquid has filled thechannels, the loop portions and the connection reservoirs, wires can beinserted into the connection reservoirs, shown in FIG. 5( h). The wirecan be used to electrically connect the elastic strain sensor to theinput electronics that will read the sensor output.

In addition to the sensors shown in FIG. 1 and FIG. 2, the pressuresensors shown in FIG. 4 can also be fabricated using layers as shown inFIG. 5. In this embodiment, the circular pattern of the pressure sensorcan be formed in Layer 1 and bonded to an unpatterned layer, Layer 0 asdescribed herein.

FIG. 6 shows one embodiment of combination or multi-mode (strain andpressure/contact) sensor according to an embodiment of the invention. Asshown in FIG. 6( a), the sensor according to this embodiment can be usedto sense strain in the X and Y dimension and pressure in the Zdimension. FIG. 6( b) shows a top view of the multi-mode sensoraccording to an embodiment of the invention wherein the twounidirectional strain sensors are arranged with their strain axesorthogonal to provide strain sensing in the X and Y dimension and apressure sensor is provided on the top layer to sense pressure in the Zdimension. FIG. 6( c) shows the individual layer patterns for themulti-modal sensor.

In accordance with one or more embodiments of the invention, amulti-modal sensor can include three soft sensor layers made of siliconerubber (FIG. 6) that is highly stretchable and soft (modulus: 69 kPa,shore hardness: 00-30). Layers 1 and 2 can include straight-linemicrochannel patterns that are sensitive to axial strains as well as tocontact pressure and Layer 3 can include a circular pattern for pressuresensing but is not sensitive to axial strain. Layer 2 can be placed ontop of Layer 1 with a 90 degree rotation for detecting strain along aperpendicular axis. Using the combination of the signals from the threesensors, the device can detect and distinguish three different stimuli:x-axis strain, y-axis strain, and z-axis pressure (see FIG. 6( a)). Allthree sensor layers can be connected through interconnects (p2 and p3 inFIG. 7) between layers, making one circuit that is electricallyequivalent to three variable resistors connected in series.

The multi-modal sensor can be fabricated using a layered molding andcasting process, as shown in FIG. 8. The process can be divided intothree steps, casting, bonding, and EGaIn injection. The base materialcan be an elastomer material, for example, silicone rubber (e.g.,EcoFlex0030, Smooth-On, Inc., Easton, Pa.), which is chosen for itscombination of high stretchability (elongation at failure: 900%) andease of casting at room temperature. A relatively low mixed viscosity(3000 cps) is an additional consideration in order to successfullyreproduce the features of the mold. The first step is to cast separatesensor layers (see FIGS. 8( a) and (b)). Plastic molds are preparedusing a 3D printer (e.g., Connex500, Objet Geometries Ltd., Billerica,Mass.), and liquid silicone is poured into the molds. The second step isto bond layers to make a single sensor structure (see FIG. 8( c)-(f)).The cured layers are bonded by spin-coating liquid silicone betweenlayers. Partial curing of the spin-coated silicone prevents the siliconefrom blocking microchannels. Also, alignment posts in the moldsfacilitate aligning the interconnects between layers. In the final step,EGaIn is injected into the microchannels and wire connections are madeby inserting electrodes (see FIGS. 8( g) and (h)). FIG. 9 shows how eachlayer is bonded to the previous layer with alignment, as described inFIG. 8( c)-(e). In each bonding step, alignment is important to ensurethe channel connection between layers through interconnects.

In accordance with one embodiment, a multi-modal sensor 100, as shown inFIG. 6, can be included as part of skin or outer covering for a movingcomponent. In this embodiment, the channel dimensions can be 200 μm by200 μm for strain sensing (Layers 1 and 2) and 200 μm (width) by 200 μm(height) for pressure sensing (Layer 3). The overall size of theartificial skin can be 25 mm by 25 mm, and the thickness can beapproximately 3.5 mm.

FIG. 10A shows the circuit diagram that can be used to read signals fromthe three sensor layers. A constant current source can be used togenerate constant current that flows through the three sensors inseries, creating voltage drops at each sensor layer. The voltagedifference across each sensor can be amplified by an instrumentationamplifier. The amplified signals can be connected to threeanalog-to-digital conversion ports of a microcontroller to separatelymeasure the resistance changes.

The multi-modal sensor can be calibrated by applying strains in multipledirections and contact pressure using, for example, a materials tester(e.g., Instron 5544A, Instron, Norwood, Mass.). In one embodiment of theinvention, the multi-modal sensor can be stretched up to 100% in both xand y axes for strain sensing (FIG. 10B), and the center of the sensorcan be compressed up to 60 kPa for pressure sensing. The results showedlinearity in strain sensing and nonlinearity in pressure sensing asshown in FIG. 11. FIG. 11 (a) shows x-axis strain, FIG. 11 (b) showsy-axis strain, and FIG. 11 (c) shows y-axis pressure However, the sensorsignal was repeatable in both cases. Since the signals from the threesensor layers displayed different responses in each experiment, theprototype is able to not only measure the magnitudes of strains andpressure but also distinguish the types of stimuli.

In accordance with one embodiment of the invention, one or more of themulti-modal sensors can be incorporated in an artificial skin thatprovides sensory response without additional sensors. These artificialskins can be used for humanoid robots [31], robotic prosthetics [34],and soft wearable robots [32], [9].

FIG. 12 shows an example of an elastic strain sensor according to anembodiment of the invention applied to measure the angular movements ofa robotic arm. The strain sensor can be attached to the joint of arobotic arm that has at least one degree-of-freedom (d.o.f.). The twoends of the sensor can be fixed to the robotic arm, and the middle partcan slides on the joint without friction. When the joint makes arotation to bend the arm, the strain sensor can become stretched aroundthe joint proportional to the arc angle. In this way, the arc angle canbe easily measured by the elastic strain sensor according to anembodiment of the invention.

The arc length can be simply calculated as Δl=rθ. Then, from equation 1,ΔR/R₀=Gε and ε=Δl/l₀ where ΔR is the resistance change, R₀ is theoriginal resistance, G is the gauge factor, and ε is the strain.Assuming there is no temperature change, the angular change (θ) can bedetermined as following:

$\begin{matrix}{\theta = \frac{l_{0}\Delta \; R}{{GR}_{0}r}} & (6)\end{matrix}$

where l₀ is the original length of the strain sensor. The strainresponse of the sensor can be determined empirically, from thecalibration experiment and shown to be linear (FIG. 3), which means G isa constant, and l_(o), R₀, and r are all constants, the angular positionof the robotic arm can be linearly proportional to the resistance changeof the sensor.

In accordance with an embodiment of the invention, motion sensing can beexpanded to measure the angular position of the robotic arm in 3Dmotions (multiple d.o.f.s) by adding more sensors to differentlocations, as shown in FIG. 13. Assuming the robotic joint in FIG. 13has only two d.o.f.s., the minimum number of sensors needed formeasuring 3D angular positions is two, although more sensor can be used.Where the sensor signal for strain change is linear, we can construct asimple matrix to calculate the joint angles such as

$\begin{matrix}{\begin{bmatrix}\theta_{xz} \\\theta_{yz}\end{bmatrix} = {C \cdot \begin{bmatrix}s_{1} \\s_{2}\end{bmatrix}}} & (7)\end{matrix}$

where are θ_(xy) and θ_(yz) are angles of the robotic arm projected toxz and yz planes, respectively, and s₁ and s₂ are sensor signals fromsensors 1 and 2, respectively. C is a calibration matrix (2×2 in thisexample), and it can be found experimentally.

In accordance with embodiments of the invention, the application of thestrain sensor is not limited to a robotic joint. The sensors accordingto the invention can be used to measure the joint angles of humanbodies. The highly soft and stretchable properties of the sensorsaccording to the invention make the sensor easily conformable tocomplicated shapes of different human bodies. FIG. 14 and FIG. 15 showexamples of applying the strain sensor to acquire joint angleinformation. In these embodiments, the multi-modal sensors according tothe various embodiments of the invention can be used to measure morecomplex joint motion than shown in FIG. 12 and FIG. 13.

The present invention can be used in a system for evaluatingbiomechanics using the flexible strain and pressure sensors describedherein.

In accordance with one embodiment of the invention, the system uses theflexible sensors to detect and measure strain, pressure, shear, andcurvature of a biomechanical system such as a joint or set of joints ofa subject under study. As described herein, the flexible sensorsincorporate micro-channels, filled with a conductive liquid metal alloyas shown in FIGS. 1, 2, 4, 6 and 7. For the strain sensor, when theflexible material experiences strain in the axial direction of themicro-channels, the overall channel length increases and thecross-sectional areas of the channels decrease, causing an increase inthe overall measured resistance. The measured resistance can becalibrated with a joint angle for this sensor in order to provide adirect measurement of the angle of the given joint with respect to oneof the limb segments. In addition, one or more pressure sensors can beprovided in an insole of a shoe worn by the subject in order to measurethe applied external forces to the environment.

In accordance with one embodiment of the invention, a modular sensorsystem can be provided whereby each joint (e.g., ankle, knee, hip,wrist, elbow, shoulder, etc.) or rigid body (e.g., hand, forearm, foot,shank, thigh) can be fitted with a separate sensor subsystem or module.Each sensor subsystem can include a flexible brace with one or moreflexible sensors, one or more processors, and one or more energy sources(battery, or motion-generated power). The user could choose to use oneor more of the sensor modules on one or more joints or rigid elements ofthe subject depending on the desired application.

In accordance with one embodiment of the invention, a hybrid bracesystem can be provided whereby each joint or rigid body can be fittedwith a subsystem or module. Each subsystem can include one or moreflexible braces, one or more flexible sensors, one or more processors,and one or more batteries, as well as one or more force sensors, bendsensors, pressure sensors, torque sensors, tilt sensors, accelerometers,gyroscopes, magnetometers, and/or optical sensors. This hybrid systemwith additional sensing modalities can be appropriate for certainapplications.

In accordance with one embodiment of the invention, a hybrid shoe systemcan be provided. The hybrid shoe system can include one or more flexiblesensors to obtain ankle angles and includes one or more additionalsensors, such as force sensors, pressure sensors, torque sensors, tiltsensors, accelerometers, gyroscopes, magnetometers, and/or opticalsensors, in order to infer stride length and running speed, in additionto the associated ankle biomechanics.

In accordance with some embodiments of the invention, the pure strainand/or hybrid (e.g., strain and pressure) embodiments can include asoft, flexible garment that serves to properly position the sensorsrelative to the desired anatomical structures of the subject. Thesegarments can include rigid support elements or structures to assist ingarment stabilization. In addition, depending on the application, thesupport elements or structure may or may not affect the range of motionof the subject. In some embodiments of the invention, the garment can beseparable from the sensors and electronics for easy wash ability.

In accordance with some embodiments of the invention, the system can beuntethered, for example, the controller and the batteries can be a partof the system worn by the subject without the need for wires to connectto a separate off-body laptop/desktop/plug-in power supply, etc.Wireless communications, such as WiFi, Blue Tooth, Zig Bee, can be usedto transfer data between the controller worn on the subject and aremotely located computer. In some embodiments of the invention, eachsensor can have an individual power, processer and transceivercomponents, and in other embodiments the sensors can be tethered (e.g.,wired or wirelessly connected) to a single electronic device that ismeant to be worn by the subject and which will provide power, processingand wireless data transmission for all sensors.

In accordance with some embodiments of the invention, the controlsignals sent to the sensor system and motion data measured by the sensorsystem can be wirelessly transmitted under software control on acomputer and transmitted to a secure data storage site.

In other embodiments, one or more sensor outputs can be input into acomputer/processor running a biomechanical model (e.g., a softwareprogram) and this model can be used to generate estimates of limbsegment motion and orientation.

In accordance with the invention, the calibration for each of theseembodiments can be activity and limb-segment dependent. Thus, acalibration routine may be needed for some or all activities that usethe sensor system. In addition, for applications, a higher-fidelitycalibration can be used, for example, for rehabilitation applications ascompared to game/computer interface applications.

FIGS. 20A and 20B show various prior art systems for evaluatingbiomechanics. FIG. 20A shows a system that uses passive or active visualmarkers that are used in making a video recording of a subjectperforming a task. The visual markers are positioned to enable aclinician, as well as software applications, to evaluate the videorecording to study the biomechanics of the subject. FIG. 20B shows asystem that uses inertial measurement devices, positioned on the subjectfor the same. The motion data data determined by the inertialmeasurement devices can be processed to evaluate motion. FIG. 20C showsa system according to the invention, wherein a flexible sensor ispositioned on the joint to measure the angle of the joint based onestrain experienced by the sensor as the joint is flexed and extended.

FIG. 21 shows an example of a system according to the invention formonitoring the forces and motion of a joint. In this example, the systemcan measure the forces experienced by the foot. In this embodiment, thesensors, force sensor 2 and kinematic sensor 3 can be mounted to aneoprene sock 1 that is worn by the subject. The neoprene sock 1 caninclude a zipper 4 for easy removal as well as one or more rigid orsemi-rigid support elements (not shown) The sensors 2 and 3 can beconnected by wires to the controller 5 that includes a power source suchas a battery. The controller 5 can communicate wirelessly tocomputerized device such as a desktop or portable computer, a smartphone or tablet computer, executing one or more software programs toreceive the sensor data and provide, for example an assessment of themotion detected.

FIGS. 22A-22E shows various examples of application for the use of thesensor system according to the invention. FIGS. 22A and 22B showrehabilitation applications. FIGS. 22C and 22D show rehabilitationapplications in the context of assessing performance. FIG. 22E is oneexample of a computer game interface application.

FIG. 22A shows an elbow rehabilitation system worn by a subject. Thesystem can transmit data to the subject's mobile phone, which canforward the exercise data to a care provider. The system can notify thesubject via a smartphone app as to when to perform rehab tasks as wellas guide the subject through the prescribed routine. FIG. 22B shows anelbow rehabilitation device with an interface for performing requiredmotions. In this embodiment, the sensor system can communicate with acomputer system (e.g., a desktop/laptop, or system embedded in a TV,smart phone or other computerized device) to enforce and monitorphysical and/or occupational therapy. In these embodiments, the sensorsystem can be embodied in a brace work over a subject's joint, such asan elbow or wrist. The sensor system can include one or more flexiblesensors mounted in brace positioned on the joint to measure changes injoint angle. The joint angle data can be stored on the controller andcommunicated to a remote device.

FIG. 22C shows a multi joint (e.g., shoulder, elbow, and wrist)monitoring system comprised of a set of braces worn on each joint (or asingle brace, like a sleeve extending from the wrist to the shoulder)that can measure the performance of each individual joint during asporting activity. The performance data can be stored locally on thedevice for retrieval after the sporting activity is completed, or it canbe wirelessly transmitted to a remote computer at various times duringthe activity. Thus, after a football game or tennis match, the subjectand/or a care giver can analyze the movement data and assess theperformance of the subject.

FIG. 22D shows a multi-joint (e.g., knee and ankle) monitoring systemcomprised of a set of braces worn on each joint (or a single brace, likea sleeve extending from the ankle to the hip) that can measure theperformance of each individual joint during a sporting activity. Theperformance data can be stored locally on the device for retrieval afterthe sporting activity is completed, or it can be wirelessly transmittedto a remote computer at various times during the activity. Thus, after arun or a soccer game, the subject and/or a care giver can analyze themovement data and assess the performance of the subject.

FIG. 22E shows a sensor system for use as a computer game interface. Inthis embodiment, sensor braces or bands can be worn by the users ontheir joints, such that motion of any monitored joint can be detected.The motion of a specific joint can be interpreted by a computer gameconsole to control some portion of the game. For example, monitoringbraces worn on wrists, elbows, knees and ankles can be used to reportjoint angles to a computer gaming system. The joint angle input can beused to evaluate the user motion, for example, dancing or running in thecontext of the game.

The sensor platforms previously described can be implemented for manyapplications, including rehabilitation, clinical motor assessment, drugdelivery assessment, biomechanics and motion analysis, computer and gameinterface, human modeling and self-evaluation for performanceimprovement.

In accordance with some embodiments of the invention, the sensor systemscan be used in rehabilitation applications, includingtele-rehabilitation applications. In accordance with the invention,patients can wear the modular unit(s) on the joint(s) that they areworking to rehabilitate to track their recovery progress. For example,after ACL surgery, a modular unit designed for the knee would be wornduring physical therapy and during rehabilitation exercises at home. Thephysical therapist can compare the amount of time exercised and theresulting performance. In cases where insurance does not cover multiplephysical therapy sessions, a therapist can check the progress of thepatient as they perform at home by logging into a secure data storagesite to which the data has been uploaded. Further, the braces can bebundled with customized computer or smartphone applications for patientrecovery. The apps provide real-time visualizations to follow aself-guided rehabilitation program and also provide real time alerts ifa patient is favoring a non-injured joint.

In accordance with some embodiments of the invention, the sensor systemscan be used in clinical motor assessment applications, for exampleclinical research. Research is ongoing in the field of electrical andmechanical assistance for improving pathologies associated with motorcontrol. The systems according the invention can be used to provideinformation regarding the patient's motor control with and without theassistive device. This additional information can be used for clinicalassessment and evaluation of the efficacy of new assistive devices.

In accordance with some embodiments of the invention, the sensor systemscan be used in drug delivery assessment applications Implantableneurological stimulators and implantable drug pumps show promise in thetreatment of a variety of diseases and ailments. Setting therapeuticlevels and dosages is still difficult because it often relies on aclinician's observation of symptoms, or a patient's self-report ofsymptoms, such as tremors, during a dosage paradigm that can take hours,weeks, or months. The sensor system embodiments according to theinvention can provide continuous monitoring of motor parameters andprovide information to assess the tuning of the drug delivery forindividuals.

In accordance with some embodiments of the invention, the sensor systemscan be used in biomechanics applications, including in the field(outside the lab or clinic) applications. Current methods for motionanalysis limit the ability to collect data outside of the labenvironment. Systems according to the invention can be used to obtainbiomechanical measurements during real situations as opposed tosimplified or simulated laboratory exercises. For example, the modularsystem according to the invention can be used to further evaluatetherapy and treatment strategy in sports medicine as the flexiblegarments encasing the sensors are similar to braces commonly worn duringgames. In addition to strategy, a more thorough understanding of sportsinjury can be developed. Currently injuries can be assessed byperforming biomechanics analyses before or after the injury. Using theembodiments described here, athletes could wear the sensing system whileplaying. Then if an injury occurs, the evaluation could be made based onthe biomechanics at the time of injury. This would be beneficial formany common injuries that are not well understood, such as runner'sknees, Achilles tendon injuries, ACL sprains and tears, etc. In additionto sports activities, injuries in other activities, such as music, danceor occupational maladies could be further understood. For example,injuries such as carpal tunnel syndrome in piano players, musclecontractures during violin playing, and knee injuries in dancers couldbe studied to improve technique and reduce injury as well as to optimizerehabilitation after injury.

In accordance with some embodiments of the invention, the sensor systemscan be used in computer and game interface applications. Many gamingsystems are moving towards motion based system control. By using themodular sensor system, an interface to a gaming system or computer canbe developed to control software programs based on the directly measuredmotions of the user. In this embodiment, the number of sensors can beminimized by selecting the least number of sensors needed for the gameapplication. Further, in a related application, a specialized computerinterface could be developed that permit a disabled user to control acomputer system using limited biomechanical functionality.

In accordance with some embodiments of the invention, the sensor systemscan be used in Human Modeling. The sensor system according to theinvention can be used to obtain information about the body incombination with other sensors or sensor systems. For example couplinginertial data with joint angle information can lead to a betterprediction of the mass and inertia properties of the body. In anotherexample, by applying a known force to the biomechanical system with thestrain sensors, one can obtain an estimate of the joint stiffness.Inversely, by applying an additional known stiffness to the joint, onecan obtain information on the dynamic force production ability of theuser.

In accordance with some embodiments of the invention, the sensor systemscan be used in personal performance system applications. There are anincreasing number of personal systems for self-evaluation, includingpedometers and accelerometers for evaluating a person's steps per day orrunning speed. The sensor system according to the invention can be partof a platform can provide individuals with personal performanceinformation for self-training and evaluation using real-time feedback.For example, marathon trainers could see how their biomechanics changedue to fatigue. Similarly skill-acquisition that requires a focus onbody posture and positioning (e.g., dance, kung-fu, tai chi, yoga, golf,basketball, football, soccer, etc.) could be improved.

Other embodiments are within the scope and spirit of the invention. Forexample, due to the nature of software, functions described above can beimplemented using software, hardware, firmware, hardwiring, orcombinations of any of these. Features implementing functions may alsobe physically located at various positions, including being distributedsuch that portions of functions are implemented at different physicallocations.

Further, while the description above refers to the invention, thedescription may include more than one invention.

The following references are cited in the text. Each of the followingreferences is hereby incorporated by reference in its entirety.

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1. An elastic strain sensor for sensing strain along a strain axiscomprising: an elastic material having two or more channels, eachchannel extending from a first end to a second end, substantiallyparallel to the strain axis; at least one loop portion connecting thefirst end of a first channel to the first end of a second channel,wherein the loop portion has a substantially large cross-sectional areaalong an axis transverse to the strain axis; and a conductive liquidextending continuously from at least the second end of the firstchannel, through the loop portion, to the second end of the secondchannel. 2-15. (canceled)